6-6 Parent-child relationship layer by layer: late binding

A. Introduction

Consider the Shape class and its three subclasses below. There are three methods in the superclass: area(), perimeter(), and toStr(). Rectangle class overrides only toStr(); Triangle class overrides all three methods, and Circle class overrides only area() and perimeter() besides providing new methods of toString(), roll() and equals(). Notice toString() does not override toStr() since they have different names.

B. Late Binding (a.k.a Dynamic Binding):

Now review the Main class which creates an array of three different shapes:

import java.util.Scanner;
class Main {
  public static void main(String[] args) {
    Shape[] three = new Shape[3];
    Scanner in = new Scanner(System.in);
    for(int i=0;i<three.length;i++){
    System.out.println("What shape do you have?(1-circle, 3-Triangle, 4-rectangle)");
    int choice = in.nextInt();
    in.nextLine();
    System.out.println(" choice: " + choice);
    if(choice==1){
      System.out.println("radius and center?");
      double r = in.nextDouble();
      double x = in.nextDouble();
      double y = in.nextDouble();
      in.nextLine();
      Point p = new Point(x, y) ;
      three[i] = new Circle(p, r);
      ((Circle)three[i]).roll();  // type casting
    }
    else if (choice ==3){
      System.out.println("three sides of the triangel?");
      double a = in.nextDouble();
      double b = in.nextDouble();
      double c = in.nextDouble();
      in.nextLine();
      three[i]=new Triangle(a, b, c);
      // Is it possible to convert a triangle into a circle? - A runtime type casting exception!
      ((Circle)three[i]).roll();
    } 
    else if (choice==4){
      System.out.println("length and width?");
      double l = in.nextDouble(); 
      double w = in.nextDouble();
      in.nextLine();
      three[i] = new Rectangle(l, w);  
    }
    System.out.println(three[i].toStr()+", area = "+three[i].area()+", perimeter = "+three[i].perimeter());
    }
  }
}

Notice three[i].toStr(), three[i].area(), and three[i].perimeter() will invoke different versions of toStr(), area(), and perimeter() depends on the ACTUAL object type of three[i].

Parent-child relationship rules #5: the following type rules must be followed for calling polymorphic methods

C. Try this:

1. Question 1-8 and 10 at end of Chapter 3 (page 150), Barron's AP CS A

2. Implement the public double weeklyPay() method in the class hierarchy below so you can solve the payroll problem as described here.

3. Create an array of animals that has 1 cat, 1 dog and 1 cow, then call public String say() to let each of them speak in their own language.